This one number decides when ski resorts can make snow

Think ski resorts just need freezing temperatures to make snow?

There’s another number resorts watch before firing up their snow machines, and it could make all the difference in when ski season begins.

(MORE: The race to open is on as Colorado ski resorts start snowmaking)

Wet-bulb temperature and skiing

When it comes to snowmaking, resorts look beyond the air temperature.

Ski resorts pay close attention to the wet-bulb temperature, which accounts for both air temperature and humidity.

A wet-bulb temperature is what a thermometer reads when its bulb is covered in a wet cloth. As water evaporates from the cloth, it cools the thermometer, much like sweat cools your skin.

It shows how cool the air can get through evaporation, and that same cooling process plays a role in snowmaking.

When the air is dry, water can evaporate more easily, allowing the remaining water to cool faster. But when the air is humid, evaporation slows down, making it harder for water droplets to freeze.

That’s why two ski resorts with the exact same air temperature could have different snowmaking conditions.

(MORE: Snowmaking is underway at Mt. Rose as Tahoe’s earliest opening approaches)

Snowmaking doesn’t require freezing air

Here’s where things get interesting.

The thermometer doesn’t always need to read 32 degrees or lower for snowmaking to begin.

Snow guns spray tiny water droplets into the air where they cool and freeze before reaching the ground.

Under dry conditions, evaporative cooling can help those droplets freeze even when air temperatures are above freezing. In some cases, resorts can make snow with air temperatures closer to the 40s.

But there’s a catch.

Most conventional snowmaking systems generally need wet-bulb temperatures around 26 and 28 degrees or colder to begin effectively producing snow.

The colder the wet-bulb temperature, the more efficiently those machines can operate and the produce more snow.

So even if the thermometer reads 32 degrees, high humidity could keep conditions too warm for snowmaking.

(MORE: Northeast ski resorts begin first snowmaking push ahead of 2026-27 season)

Humidity decides opening day

Why do some ski resorts manage to open in October, sometimes before they’ve seen sufficient natural snowfall?

The answer is humidity.

Mountain resorts in Colorado and Utah often benefit from drier air which can create favorable snowmaking conditions even when temperatures are slightly above freezing.

Meanwhile, East Coast resorts can face challenges when humid air moves in, making it harder to reach ideal wet-bulb temperatures without colder air.

And not all Western resorts have this advantage. Resorts in Sierra Nevada and the Pacific Northwest can also experience humid conditions when moisture moves in from the Pacific.

Ultimately, it’s just how cold it gets but how dry the air is that can make all the difference in getting the ski season started.

Faiza Mohammed is a meteorologist intern at weather.com. She studied geography and worked with the Weather Dawgs team at the University of Georgia. She also received her master’s in Earth and Atmospheric Sciences from the Georgia Institute of Technology.